Why can I not keyframe this armature?
If you look down at your time line you will see that the active keying set is "Button Keying..."

That is a custom keying set, you can see what is included in it in the scene tab of the properties window.

You can do one of two things.


Add the location and rotation to your custom keying set.  
Click on the X next to the keying set's name to clear the active keying set. (it will still be there, just not used) Then the familiar keyframe menu will pop up when you press I.